1. Field of the Invention
The present invention relates to Nb.sub.3 Al group superconductors, particularly, to Nb.sub.3 Al group superconductors and manufacturing method thereof, Nb.sub.3 Al group superconductive precursory compositions, and superconducting magnets for generating high magnetic field preferable for nuclear fusion apparatus, nuclear magnetic resonance apparatus, and high magnetic field generating apparatus all of which require high magnetic field.
2. Description of the Prior Art
Nb.sub.3 Al group superconductors have been manufactured, as described in Applied Physics Letters, vol. 52 No. 20 p 1724-1725 (1988. 5.16), in a manner that a plurality of aluminum alloy core materials and niobium matrix material are made to an ultrafine multicore cable by a composite manufacturing method, and subsequently, the cable is treated with diffusion heat treatment at 750.degree.-950.degree. C.
Another manufacturing method in which Nb.sub.2 Al powder and Nb powder are mixed, fabricated, and sintered at 1,300.degree.-1,400.degree. C. to be Nb.sub.3 Al is disclosed in The Proceedings of 45th on Cryogenics and Superconductivity (Chiba, Japan, May 14-16, 1991), p 246.
Further, other manufacturing method is disclosed in JP-A-3-283322 (1991), in which a composite body is formed by filling mixed powder of Nb.sub.2 Al alloy and niobium into a metallic tube made from niobium or Nb base alloy, a core material is formed by cold working of the composite body, and subsequently, Nb.sub.3 Al superconductor is obtained by heat treatment of the core material for generating Nb.sub.3 Al which changes the cold worked powder to Nb.sub.3 Al.
Among the above described prior art, the composite manufacturing method had a problem that the Nb.sub.3 Al ultrafine multicore cable obtained by normal diffusion heat treatment had low critical temperature and low critical magnetic field, and the critical current density decreased remarkably under a high magnetic field condition such as 20 T level. The method by fabrication and sintering of Nb.sub.2 Al powder and Nb powder necessitated sintering at high temperature, and accordingly, it had a problem although it enhanced yielding of Nb.sub.3 Al that crystal grain size of Nb.sub.3 Al increased and the critical current density decreased under low and medium magnetic field conditions.
That means, a case of Nb.sub.3 Al group superconductors, high critical temperature and high critical magnetic field depending on characteristics of Nb.sub.3 Al material itself can not be realized, and accordingly, the critical current density under a high magnetic field condition is low, and the Nb.sub.3 Al superconductors do not become cable materials for practical use. Generally speaking, it is well known that the critical current density of Nb.sub.3 Al group superconductor depends on size of the crystal grain, although a Nb.sub.3 Al superconducting phase by the above described conventional composite manufacturing method includes fine crystal grains and many magnetic flux pinning centers which are effective under a relatively low magnetic field condition, the Nb.sub.3 Al group superconductors have low critical current density under a high magnetic field condition because of shifting the composition ratio of niobium and aluminum etc.
Further, it is well known that Nb.sub.2 Al non-superconductive phase acts as an effective magnetic flux pinning center under a high magnetic field. However, the conventional diffusion reaction of niobium and aluminum generates aluminum enriched NbAl.sub.3 phase first in accordance with thermal equilibrium, and subsequently generates Nb.sub.2 Al phase and superconductive Nb.sub.3 Al phase through Nb.sub.2 Al phase. Therefore, a great effort is devoted to form Nb.sub.3 Al single phase, but Nb.sub.2 Al non-superconductive phase does not exist in the single phase.
Further, various methods for generating only superconductive Nb.sub.3 Al phase having a large amount of grain boundaries which are magnetic flux pinning centers in Nb.sub.3 Al phase, that means fining the grain size, have been attempted, but any method has not succeeded. Moreover, a method in which a diffusion reaction is performed at high temperature with Nb.sub.2 Al phase and niobium as starting materials was proposed. But, although the proposed method relatively facilitates formation of Nb.sub.3 Al phase, grain size remains still large. The method disclosed in JP-A-3-283322 (1991) neither indicate any improvement on critical current density under a high magnetic field condition, nor any special consideration on a relationship between the Nb.sub.2 Al non-superconductive phase and the magnetic flux pinning centers under a high magnetic field condition.